CHAPTER XVI 



THE ACTIVATION OF GENES BY THE CYTOPLASM 



IT HAS frequently been argued that genes control only the later-developed 

 and more superficial characters of animals and that the development of 

 the basic plan of the body is controlled, not by them, but by the cyto- 

 plasm of the egg; and this contention has been hotly disputed by genetic- 

 ists w^ho seem to feel that it disparages the importance of their subject. 

 We realise now that, as in so many such controversies, both sides are in 

 the right. Undoubtedly within any one lifetime a great deal of the basic 

 pattern of the body is dependent on the configuration of the cytoplasm of 

 the egg; one need only remember such phenomena as the arrangement 

 of ooplasms in mosaic eggs (p. io6), of gradients in the echinoderms (p. 

 85), or of the formation centre in insects (p. 126). There are indeed many 

 cases in which genetic differences in the nuclei can be shown to have an 

 influence even in early stages of development (e.g. in the merogons or 

 hybrids in frogs, p. 358), but they are certainly not all-important. Thus 

 for embryology the cytoplasm is as fundamental as the genes. 



Equally, of course, one must not forget that an egg will not develop 

 into even the general framework of an animal unless it is provided with 

 nuclei. There can be no doubt that differentiation results from the inter- 

 action of the division-products of the original zygotic nucleus with the 

 already-present ooplasmic regions of the egg. Neither cytoplasm nor 

 nucleus can be disregarded: in fact the most important subject to discuss 

 is how they affect each other. 



We can, then, put into the following form the knowledge with which 

 we have to approach the problem of how development is brought about: 



(i) There are local differences with the cytoplasm of the newly fertil- 

 ised egg. The nuclei, with the genes contained in them, react with the 

 cytoplasm with which they are in contact; and the interacting system of 

 nucleus and cytoplasm may also be affected by substances diffusing from 

 neighbouring regions, as in the process of induction. 



(2) As a result of the nucleus-cytoplasm reactions there are formed, 

 firstly new duplicate genes (which allow for the multiplication of nuclei), 

 and also substances of some kind which pass into the cytoplasm and modify 

 it. 



(3) These 'immediate gene products' may interact with each other in 



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